Syria's main Kurdish party appealed on Wednesday to world powers to stop a threatened Turkish attack on the Afrin region in northern Syria, as the US reportedly said it had no ties with Kurdish forces in the enclave.

Ankara has warned in recent days of an imminent military operation in Afrin, one of three autonomous cantons under the control of Kurdish forces and their allies.

On Wednesday, Turkey said it will respond immediately to any threats from Western Syria that could harm the country or its citizens, according to the the National Security Council (MGK).

Turkey added it would not hesitate to take action in Afrin and other areas unless the United States withdrew support for a Kurdish-led force there, as the Pentagon downplayed its plans to train the Syrian fighters.

Speaking to reporters after a cabinet meeting, Deputy Prime Minister and Government Spokesman Bekir Bozdag said the planned US-backed force posed a threat to Turkey's national security, territorial integrity and the safety of its citizens.

"We emphasised that such a step was very wrong," he said. "Turkey has reached the limits of its patience, nobody should expect Turkey to show more patience."

In a statement, the Pentagon said it was training "internally focused" Syrian fighters with a goal of preventing the Islamic State (IS) group's resurgence and ensuring Syrians displaced by the war could return to their communities.

"This is not a new 'army' or conventional 'border guard' force," the Pentagon said, adding it would be "completely transparent" with Turkey about its plans.

"We are keenly aware of the security concerns of Turkey, our coalition partner and NATO ally. Turkey's security concerns are legitimate," it said.

In a statement, the MGK said it would not allow the formation of a "terrorist army" along Turkish borders, after the US-led coalition in northern Syria said it was working with the mainly Kurdish YPG-led Syrian Democratic Forces (SDF) to set up a new 30,000-strong border force.

The MGK demanded that all weapons given to Kurdish fighters in Syria be collected without delay. It also advised the cabinet of ministers to extend the emergency rule imposed shortly after a 2016 coup attempt.

The appeal by the PYD in Afrin came hours after the US was reported as saying it had no involvement with Kurd authorities there, as they were not part of the US coalition's area of operations against the Islamic State (IS) group.

"The coalition's mission has not changed: to defeat ISIS in designated areas of Iraq and Syria and set conditions for follow-on operations to increase regional stability," US army Colonel Ryan Dillon was quoted as saying by the Turkish state news agency Anadolu. "We are not operating in Afrin."

Adrian Rankine-Galloway, a US defence department spokesman, told Anadolu: "We don't consider them as part of our 'defeat ISIS' operations, which is what we are doing there and we do not support them. We are not involved with them at all."

Turkey was reported to have moved 10 tanks to the border near Afrin on Tuesday, and was moving additional forces towards the area.

In its statement, the PYD said Afrin "will not be alone" and that all of northern Syria would aid its defence.

"We call on the international community... to take responsibility towards more than a million people living in Afrin," the PYD said.

"The Turkish regime... has become a threat to any solution to the Syrian crisis," it added.

Since the conflict began in 2011, the Syrian Kurdish YPG militia and its allies have set up autonomous cantons in the north of the country. Their sphere of influence expanded after they joined forces with the United States to fight Islamic State - although Washington opposes their autonomy plans.

The Turkish regime... has become a threat to any solution to the Syrian crisis

- PYD statement

The party - the YPG's political affiliate - urged the United Nations to take immediate action to turn Kurdish-led parts of northern and eastern Syria into a safe zone.

The YPG and Turkish forces inside Syria have accused each other of shelling around the Afrin region in recent days.

Turkey views the YPG as an extension of the outlawed Kurdistan Workers' Party (PKK) which has fought a campaign in southeast Turkey since 1984.

The US-led coalition said this week that it is building a new 30,000-strong border force with its Syrian militia allies in the north, a plan that has infuriated NATO ally Ankara.

The Turkish president, Recep Tayyip Erdogan, pledged to crush the new force before it came into existence.

Erdogan on Tuesday contacted the Nato secretary-general, Jens Stoltenberg, to say Turkey would take all necessary precautions to ensure its national security, presidential sources said.